HK97 are mature protein particles, assembled as provirions in an E. coli system expressing the 42kD capsid protein of HK97 and induced to maturation by exposure to 6M urea. To our knowledge these are the first crystals of a dsDNA phage that diffract x-rays. Our need for synchrotron radiation results from the low intrinsic scattering from such a large unit cell (and the relatively few number of them in the crystal), thus the intensity is required to give us the highest resolution patterns.